Is the color of your glaze a product of the chemistry? (e.g. does it require a certain amount of CaO to develop a chrome-tin pink color) If so this means that Boraq is either lacking or supplying an oxide or mineral that GB had or did not have. If you are using a stain look through the stain manufacturers brochure to see what notes they have for the color system you are using. We have formulated Boraq 1 to be a little short on CaO, MgO and SiO2 (for reasons explained elsewhere). If colors depend on adequate CaO, for example, then you need Boraq 2 (it has added dolomite and whiting). If your color depends on silica, a simple addition will fix this. Boraq 1 also contains a little more B2O3 than GB; if color is affected by this, using less Boraq might be the solution.
Is the color a result of crystal development (e.g. a very fine mesh of iron-silicate crystals can turn a normally glossy black glaze into a matte yellow one). Crystals normally grow in more fluid glazes and GB was a great material to make fluid glazes. If you are using Boraq 1 your glaze will likely be more fluid than with GB (especially at higher temperatures) and thus it could develop more crystals than before. Boraq 2 might make the glaze less fluid and thereby develop fewer crystals.
Is the color wrong because the thickness is different? Many glazes are sensitive to thickness, especially those lacking an opacifier. Some glazes crystallize more when they are thick. The Boraq version of your glaze might be going on thicker or thinner; you will need to learn how to use it to achieve the right thickness.
Do not hesitate to use a little more or less of the colorant to restore the shade if this is all that is needed.
Glazes are normally glossy either because they have high silica or boron glass former, low alumina, or because they are fired high enough to form a fluid melt that does not to crystallize during cooling (either because of the right chemistry or because cooling is quick enough not to give the glaze a chance to crystallize). If the Boraq version of your glaze is more matte is it because it is forming a crystal mesh on the surface? Boraq 2 melts it less than Boraq 1. If this takes it in the right direction but not far enough, try using less Boraq 2 or add a little kaolin (to supply silica and alumina and thereby reduce flux proportions). If the Boraq 2 version of the glaze is more matte because it is not melted enough, then use more Boraq 2. If the Boraq version is glossier then try to determine if this is happening because it is melting more (a flow test is perfect for this). If so, use less Boraq or use Boraq 2 instead of Boraq 1, or add a small amount of kaolin. If it is glossier because it is forming fewer crystals, you will have to identify the mineralogy of the crystals to determine type of chemistry they require to develop.
Gerstley Borate glazes often had textures that were partially due to the bubbling activity produced in the melt as a result of gases of decomposition escaping. Boraq has about the same LOI, but the gases may escape at different temperatures (and therefore different stages in the melt). Small dimples (or even pinholes) in the surface of thick applications can occur with GB or Boraq based glazes. If you either want or don't want this effect you need to determine at what temperature it is occurring at and adjust the firing to compensate. If you need to develop a flawless glaze surface for large production you may need to have a thermogravimetric analysis done, this tells you at what temperature the glaze is gasing off and how much. You can then tailor your firing curve to slow down at maximum output times and speed up at other times (remember that because Boraq and GB are composed of a mixture of minerals they expel gases at different stages of fired as each mineral decomposes).
GB glazes often had blistering problems because of the high LOI and the fluidity of the melt. Often times companies learned how to live with some blistering problems to get the visual effect they wanted. Boraq, for whatever reason, might be worse or better. Check the Digitalfire blistering troubleshooting page on this for more information on solution strategies.
See the sections above on texture and color variations for more information on this. As mentioned, a flow tester is the ideal way to determine if you are getting the same melt using Boraq. If the glaze is melting too much, try reducing the amount of Boraq (and vice versa). If this does not work try adding a little kaolin to reduce melting; or remove a little kaolin or silica to improve it.
Raku glazes were up to 80% Gerstley Borate (this gave the slurry a gelling character unlike any other). In Raku the name of the game is melting. Temperatures are low and you want the glaze to melt properly to form the desired metallic effects. A cone 06 and lower, GB melts better than Boraq 1. Thus you might try using more of the latter or adding a little low melting frit like Ferro 3134.
Recipes high in Gerstley Borate had a habit of crawling (during drying on the ware they would shrink and pull away from the body). If you are using a special effect glaze that is intended to crawl then Boraq will likely be different since it does not shrink as much and it does not require as much water in the aged slurry. To retain the effect you will have to add a little bentonite.
It was common to use GB in glazes containing very high amounts of lithium, titanium, barium, strontium, rutile, or colorants like manganese or copper. Generally this was done to produce rough surface textures or visually striking crystalline, metallic or radical color effects (these glazes are not used on functional ware of course). These glazes are likely going to be the most difficult to duplicate using Boraq. This is because these effects are not stable. Rather, they depend of very specific chemistries and small variations change the surface much more than with typical glazes (these variations often occurred with different batches of GB also). If the glaze is melting too much, try reducing the amount of Boraq (and vice versa). If this does not work you will have to have enough knowledge to identify the mechanism of the effect you are trying to achieve, then adjust the chemistry in the Boraq version to bring it back into line.
GB was very popular in mottled and variegated glazes. Floating blue is an excellent example, it has 6 different mechanisms of variegation. If the Boraq version of your glaze does not have the same variegation character, then examine it closely to try to identify the mechanisms of the various factors producing the variegation. For example, if the variegation due to bubble escape is less, then you need to modify the firing to cool more quickly through the temperatures at which bubbles are produced so their remnants are not smoothed over. If crystallization is more or less, see above. If coloration is different you might need to augment with a little stain or remove a little of a colorant. If development of the effect is critically dependent on achieving a specific viscosity in the melt, then do a flow test comparison and adjust flux balance so that the Boraq version has the same flow. If variations in thickness are important then adjust application methods.
Glazes high in GB generally did not craze. Since Boraq 1 and 2 are a little lower in silica, there is a possibility that glazes that were formerly borderline could craze. If a good melt is being achieved the simplest solution is to add a little silica. If not, you might have to investigate supplying part of the boron from a frit or using a higher amount of Boraq. If there is significant K2O or Na2O in the recipe you might consider switching some of it for a lower expansion flux. For more information visit this page here.
Boraq slurries behave differently in spraying equipment. Thus you may get a
different density in the laydown and thus a different thickness or more
entrained air bubbles. Both of these factors can play a big part in the
development of subtle surface characteristics.